DE MONARCHIA OF DANTE ALIGHIERI
CHAPTER XIII: He who is hest adapted for ruling is the best director of other men.
1. He who is capable of the best qualification for ruling can best qualify others. In every action the chief intent of the agent, whether it act by necessity of nature or by choice, is to unfold its own likeness;  whence it is that every agent, in so far as it acts in this way, delights in action. Since every existent thing desires its existence, and since an agent in action amplifies its existence to a certain extent, delight necessarily ensues, for delight is bound up in the thing desired.  Nothing can act, therefore, unless existing already as that which the thing acted upon is to become; and therefore the Philosopher states in his writings of simple Being: "Every reduction from potentiality to actuality is accomplished by an actuality of like kind;"  for if anything attempted to act under other conditions, it would try in vain. Thus may be destroyed the error of those men who believe by speaking good and doing evil they can inform others with life and character; and who forget that the hands of Jacob, though false witnesses, were more persuasive than his words, though true.  Hence the Philosopher to Nicomachus: "In matters of passion and action, words are less trustworthy than deeds."  And hence the message from heaven to the sinner David: "What hast thou to do to declare my statutes?"  As if it had said, "In vain thou speakest, being other than thy words." From which we may gather that he who would best qualify others must himself be supremely qualified.
2. That only a Monarch can be supremely qualified for ruling is thus proved. Everything is more easily and perfectly adapted to any state or activity as there is present in it less of opposition to such adaptation. So those who have never heard of philosophy come more easily to a comprehension of philosophic truth than those who have heard often thereof, but are imbued with false opinions. So Galen  says with right: "Such men need double time for gaining knowledge." Now, as was shown above, a Monarch can have no occasion for cupidity, or rather less occasion than any other men, even other princes,  and cupidity is the sole corrupter of judgment and hindrance to Justice; so the Monarch is capable of the highest degree of judgment and Justice, and is therefore perfectly qualified, or especially well qualified, to rule. Those two qualities are most befitting a maker and executor of the law, as that holiest of kings testifies by his petition to God for the attributes meet for a king and the son of a king, praying: "Give the king thy judgments, O God, and thy righteousness unto the king's son." 
3. It was rightly assumed, then, that the Monarch alone is capable of supreme qualification to rule. Hence the Monarch is best able to direct others. Therefore it follows that for the best ordering of the world, Monarchy is necessary.
1. Conv. 3. 2. 2: "Each effect contains something of the nature of its cause." L. c. 3. 14. 1: "For the virtue of one thing to descend upon another, that other thing must be brought to the first one's likeness; as we see plainly in all natural agencies, whose descending upon passive things brings them to resemble those agencies in so far as they are capable of so doing." So in Conv. 4. 22. 4. And see note 5, De Mon. 1. 2.
2. Conv. 2. 9. 2: "Every cause loves its effect."
3. Metaphys. 8. 8. See Par. 29. 34: "Pure potency held the lowest place; in the midst clasped potency with act such a withe as never is untwisted." S. T. 1. 54-59: 1-2. 3. 2. Also note 10, De Mon. 1. 3.
4. Gen. 27. 22: "The voice is Jacob's voice, but the hands are the hands of Esau." Cf. De Mon. 2. 12. 5.
5. Eth. 10. 1. 3: "Arguments about matters of feeling and action are less convincing than facts."
6. Ps. 50.16. Note that the "sinner" may yet be "holiest of kings" in the following paragraph. See article on David, Toynbee's Dante Dictionary.
7. Claudius Galen (130-200 A.D.), the celebrated physician of Pergamum in Asia, was up to the sixteenth century the most famous physician of antiquity with the exception of Hippocrates. Some eighty-three treatises, medical and philosophical, written by him are still extant. See Inf. 4. 143. The quotation about the difficulty of unlearning false knowledge is from De Cognoscendis Animi Morbis, c. 10.
8. De Mon. 1. 11. 5.
9. Ps. 72. 1. Par. 13. 94: "I have not so spoken that thou canst not well see that he was a king who asked wisdom, to the end that he might be a competent king."